A pipe bending machine is commonly employed to bend pipe for installation in a pipeline or other use. To perform the bending operation, the pipe is typically secured in the machine by a fixed pinup shoe, a fixed die and a movable stiffback. Hydraulic cylinders are used to move the stiffback to bend the pipe about the bending die to the desired radius of curvature. The bending die typically has a compound surface, curved about the radius of the pipe and along its length to the desired curvature. The bending dies are typically interchangeable in the bending machine which permits the machine to be adapted for bending varied diameter pipe.
In many applications, it has become common to employ pipe having an insulation coating on its outer surface. One common application is the use of coating on pipe employed in artic regions. This coating insulates the pipe against ground frost heave and potential failure of the pipe. Another common use is in the transportation of heated oil for use in heat exchangers. One such coating is polyurethane. A 48 inch diameter pipe may, for example, have a five inch thick coating of polyurethane foam.
The bending of pipe having a compressible insulating material such as polyurethane thereon in a conventional bending machine has created several problems. The conventional bending die is designed to contact a large area of the pipe being bent. Therefore, the bending die tends to push the compressible material inward over a large surface area, causing adjacent areas of the material to balloon out and break away from the pipe. In addition, the compressibility of the coating material often uses much of the stroke of the hydraulic cylinders on the bending machine to merely compress the material without bending the pipe. In several applications, the upper surface of the bent pipe adjacent the bending die has been wrinkled from the compression effects of bending. This may be attributed to the failure of the bending die to adequately support the pipe during bending through the compressible material.
Therefore, a need has arisen for an apparatus and method for bending pipe having compressible material thereon without destruction of the compressible material or damage to the pipe.